How Do Black Holes Interact and Merge With Each Other?

Updated on July 26, 2016

Black holes are one of nature’s best engines of destruction. They eat and tear apart anything within its gravitational grasp into ribbons of matter and energy before finally consuming it beyond the event horizon. But what happens when more than one of these engines of devastation meet? The Universe may be a vast place but these encounters do happen and frequently with fireworks.

Black Hole Binaries

While finding black holes has become an easier task, locating two of them in proximity to each other is not. In fact they are quite rare. Pairs that have been observed orbit each other at a distance of a few thousand light-years but as they fall closer to each other they will eventually have just a few light years separating them before merging. Scientists suspect that this is the main growth method for black holes as they become supermassive and the best method for finding gravity waves, or displacements in the fabric of space-time (JPL “WISE”). Unfortunately, observational evidence has been difficult at best but by exploring the potential physics of such a merger we may gather clues as to how they will look like and what we need to look for.

The Physics of Binary Black Hole Mergers

All black holes are governed by two properties: their mass and their spin. Technically, they might have a charge also but because of the high-energy plasma they whip up around them it is likely that they have a charge of zero. This helps us greatly when trying to understand what happens during the merger but we will need to use some mathematical tools to fully delve into this strange land with other unknowns. Specifically, we need solutions to Einstein’s field equations for space-time (Baumgarte 33).

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Unfortunately, the equations are multivariable, coupled (or interrelated), and contain partial derivatives. Ouch. With items to solve for including (but not limited to) a spatial metric tensor (a way to find distances in three dimensions), the extrinsic curvature (another directional component related to the derivative of time), and the lapse and shift functions (or how much freedom we have in our set of coordinates of space-time). Add to all of this the nonlinear nature of the equations and we have one big mess to solve. Fortunately, we have a tool to help us: computers (Baumgarte 34).

We can have them programmed so that they can approximate partial derivatives. They also used grids to help construct an artificial space-time in which objects can exist. Some simulations can show a temporary circular stable orbit while others use symmetry arguments to simplify the simulation and show how the binary operates from there. Specifically, if one assumes that the black holes merge directly i.e. not as a glancing blow, then some interesting predictions can be made (34).

And they will be important to filling in what our expectations are for a black hole binary merger. According to theory, three stages will likely occur. They will first start falling into each other in a nearly circular orbit, producing greater amplitude gravity waves, as they get closer. Secondly, they will fall close enough to start merging, making the greatest gravity waves seen yet. Finally, the new black hole will settle into a spherical event horizon with gravity waves at nearly zero amplitude. Post-Newtonian techniques such as relativity explain the first part well, with simulations based on the aforementioned field equations helping with the merging stage and black hole perturbation methods (or how the event horizon acts in response to changes in the black hole) all together give meaning to the entire process (32-3).

So enter the computers to assist with the merging process. Initially, the approximations were only good for symmetric cases but once advances in both computer tech and programming were achieved then the simulators were better able to handle complex cases. They found that asymmetric binaries, where one is more massive than the other, exhibit recoil that will take the net linear momentum and carry the merged black hole on the direction that gravitational radiation is taking. The simulators have shown for a pair of spinning black holes that the resulting merger will have a recoil velocity of over 4000 kilometers a second, fast enough to escape most galaxies! This is important because most models of the universe show galaxies growing by merging. If their central supermassive black holes (SMBH) merge then they should be able to escape, creating galaxies without a central bulge from the pull of the black hole. But observations show more bulge galaxies than the simulators would predict. This likely means that the 4000 kilometers per second is the extreme recoil velocity value (Baumgarte 36, Koss).

Another interesting prediction arose from the spin of the binaries. The rate at which the resultant black hole would rotate depends on the spins of each prior black hole as well as the death spiral they fall into, so long as gravitational energy is low enough not to cause a significant angular momentum. This could mean that the spin of a large black hole may not be the same as the previous generation, or that a black hole emitting radio waves could switch direction, for the position of the jets depends on the spin of the black hole. So, we could have an observational tool for finding a recent merger! (36) But for now, we have only found binaries in the slow process of orbiting. Read on to see some notable ones and how they may potentially hint at their own demise.

WISE J233237.05-505643.5 | Source

The Dynamic Duos

WISE J233237.05-505643.5, which is 3.8 billion light-years away, fits the bill for examining black hole binaries in action. Located by the WISE space telescope and followed up by the Australian Telescope Compact Array and the Gemini Space Telescope, this galaxy had jets which act oddly by acting more like streamers than fountains. At first the scientists thought it was just new stars forming at a fast rate around a black hole but after the follow-up study the data seems to indicate that two SMBHs are spiraling in to each other and will eventually merge. The jet coming from the region was off-kilter because the second black hole was pulling on it (JPL “WISE”).

PG 1302-102 is likely a location where a black hole merger will happen (but in actually has, since we are looking into the past with far-away objects courtesy of the nature of space and light). A quasar (or actively feeding black hole) resides there and was found using the Catalina Real-time Transient Survey (active since 2009 and making use of 3 telescopes) and data from as far back as 1993. The Survey hunted 500 million objects over a span of 80% of the sky. That activity can be measured as an output of brightness, and 1302 displayed a pattern that models indicate would arise from two black holes falling into each other. 1302 had the best data, showing a variation with corresponded with a period of 60 months. Scientists did have to make that the changes in brightness were not caused by a single black hole’s accretion disc and the precession of the jet lined up in an optimal way. Fortunately, the period for such an event is 1,000 – 1,000,000 years, so it wasn’t difficult to rule out. Out of 247,000 quasars that were seen during the study, 20 more may have a pattern similar to 1302 (Betz, Rzetelny 08 Jan. 2015, Carlisle).

Now, both of those were easy to spot because they were active, or had enough material around them to emit X-rays and be seen. What about quiet galaxies? Can we hope to find any black hole binaries there? Fukun Liu from the Peking University and team have found such a pair. They witnessed a tidal disruption event, or when one of the black holes caught a star and shredded it apart, releasing X-rays in the process. So how did they see such an event? After all, space is big and those tidal events are not common. The team made use of the XMM-Newton as it continuously looked at the sky for bursts of X-rays. Sure enough, on June 20, 2010 XMM spotted one in SDSS J120136.02+300305.5. It matched a tidal event for a black hole initially but then did some unusual things. Twice during the full period of luminosity, the X-rays faded out and the emissions fell to zero then reappeared. This matches simulations which show a binary companion pulling on the X-ray stream and deflecting it away from us. Further analysis of the X-rays revealed that the main black hole is 10 million solar masses and the secondary is 1 million solar masses. And they are close, about 0.005 light-years apart. This is essentially the length of the solar system! According to the aforementioned simulators, these black holes got 1 million more years before the merging occurs (Liu).

SDSS J150243.09+111557.3 | Source

The Terrific Trios

If you can believe it, a group of three close-proximity SMBHs have been found. System SDSS J150243.09+111557.3, which is 4 billion light years away based on a red shift of 0.39, has two close binary SMBHs with a third close in tow. It was initially though to be a singular quasar but the spectrum told a different tale, for the oxygen spiked twice, something a singular object shouldn't do. Further observations showed a blue and red shift difference between the peaks, and based on that a distance of 7,400 parsecs was established. Further observations by Hans-Rainer Klockner (from the Max Planck Institute for Radio Astronomy) using the VLBI showed that one of those peaks was actually two close radio sources. How close? 500 light years, enough to have their jets intermingle! In fact, scientists are excited at the possibility of using them to spot more systems like this one (Timmer, Max Planck).

PG 1302-102: The Final Stages Before a Merger?

As mentioned earlier, black hole mergers are complicated and often require computers to help us. Wouldn't it be great if we had something to compare to theory? Enter PG 1302-102, a quasar which is exhibiting a weird repeating light signal which seems to match what we would see for the final steps of a black hole merger where the two objects get ready to meld. They may even be 1 millionth of a light year apart, based on archival data showing that indeed the roughly 5-year light cycle is present. It would appear to be a black hole pair about 0.02 to 0.06 light years apart and moving at about 7-10% the speed of light, with the light being periodic because of the constant tugging of the black holes. Amazingly, they move so fast that relativistic effects on space-time pull the light away from us and cause a dimming effect, with an opposite effect occurring when moving towards us. This in conjunction with the Doppler effect results in the pattern we see. However, it is possible that the light readings could come from an erratic accretion disc, but data from Hubble and GALEX in several different wavelengths over 2 decades points to the binary black hole picture (California, Rzetelny 24 Sept. 2015).

Gravity Waves: A Door?

And finally, it would be negligent if I didn't mention the recent findings from LIGO on the successful detection of gravitational radiation from a black hole merger. We should be able to learn so much about these events now, especially as we collect more and more data. Exciting times!